Film carrier and bonding method using the film carrier

ABSTRACT

A film carrier according to this invention comprises a base film, a plurality of conductors formed on the base film, and a plurality of layers formed on the conductors, each being made of a low-melting point bonding metal. A bonding method of this invention comprises the steps of aligning the bonding metal layers of the film carrier with the terminals of an electronic part, and half-melting the bonding metal layers by thermo-compression, thereby to electrically connect the conductors of the film carrier to the terminals of the electronic part. The object of the present invention is to provide a film carrier which can serve to connect electronic parts to a circuit by means of a low pressure and at a low temperature, without causing damage to the electronic parts such as semiconductor elements, and also a bonding method using this film carrier.

BACKGROUND OF THE INVENTION

The present invention relates to a film carrier for simultaneouslyconnecting external leads to the electrodes of electronic parts such assemiconductor elements, and also to a bonding method using this filmcarrier.

Recent times have seen the manufacture of semiconductor integratedcircuits of increasing integration density, as a result of the progressmade in improving the manufacturing techniques. This has made itpossible to manufacture integrated circuits each of which now has morethan 100 terminals. Progress in manufacturing has, not surprisingly,been accompanied by the demand for the development of a highly efficienttechnique for bonding the terminals of these highly integrated circuitsto a circuit board. The so-called film carrier, which can help to bondthe numerous terminals of a semiconductor integrated circuit element, ata time, to a circuit board, is now attracting much attention.

The film carrier comprises a base film made of resin, on which copperconductors are formed. While being pressed onto the terminals of thesemiconductor element, the copper conductors are fused to the terminalsof a semiconductor element. More specifically, bumps made of alamination of titanium, nickel, palladium, and gold are formed on thealuminum terminals of the semiconductor element, and tin layers aredeposited on the copper conductors formed on the base film. Both thebumps and the tin layers are heated to a high temperature ranging from450° to 500° C., and the conductors are pressed onto the aluminumterminals, with a high pressure ranging from 200 to 1000 kg/cm² (20 to100 g for one 100 μm×100 μm terminal), thereby forming a gold-tineutectic alloy. This eutectic alloy bonds the conductors to theterminals of the semiconductor element.

Among the various known bonding methods using film carriers is the tapeautomated bonding method, which is disclosed in Japanese PatentDisclosure Sho 57-152147. Using this method, gold bumps are transferredonto tin layers on the copper of a film carrier, and these leads arefused to the aluminum terminals of a semiconductor element, while beingpressed onto the terminals.

The conventional bonding methods using a film carrier, however, havedrawbacks.

Since the leads are heated to a high temperature and pressed onto theterminals of the semiconductor element with a high pressure, there isthe possibility that the semiconductor element will be damaged. Further,when the conventional method is used when manufacturing colorliquid-crystal displays (e.g., a-SiTFT displays), the organic colorfilters used in the displays will be heated to a temperature over theirmaximum operating temperature of 150° C., while the leads of the filmcarrier and the terminals of the semiconductor elements used in thedisplays are being heated to the above-mentioned high temperature andhigh pressure. Consequently, this will result in the degradation of thecolor filters.

When the bonding method using a film carrier is used to bond the outerleads, such as the drive-signal terminals of a liquid-crystal device,these terminals and the leads of the film carrier are plated with asolder alloy, and the plated solder alloy is melted. The plated layersof the solder alloy may exfoliate from the terminals and the leads, ormay develop cracks, when they are cooled from the high bondingtemperature to room temperature, due to the thermal strain resultingfrom the difference in the respective thermal expansion coefficients ofthe terminals of the liquid-crystal device, the terminals being mountedon a glass plate, and the leads of the film carrier on which the deviceis mounted.

The more terminals the semiconductor element has, the more difficult itis to make them of the some height. In most cases, some terminals willbe taller than others. Consequently, when the leads of a film carrierare pressed onto the bumps formed on the terminals of a semiconductorelement, the pressure is concentratedly applied on the taller terminals.This may cause damage to the semiconductor element.

When tape automated bonding method is employed, the film carrier must bealigned with the substrate in order to transfer the bumps onto the leadsof the film carrier. Thereafter, the film carrier with the transferredbumps must be aligned with an outer terminal. In other words, twoalignment apparatuses are required, in order to perform tape automatedbonding. Since an alignment apparatus is very expensive, the tapeautomated bonding method is, therefore, uneconomical.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a film carrier whichcan serve to connect electronic parts to a circuit by use of a lowpressure and at a low temperature, without causing damage to theelectronic parts such as semiconductor elements, and also to provide abonding method using this film carrier.

A film carrier according to this invention comprises a base film, aplurality of conductors formed on the base film, and a plurality oflayers formed on the conductors, each being made of a low-melting pointbonding metal. A bonding method of this invention comprises the steps ofaligning the bonding metal layers of the film carrier with the terminalsof an electronic part, and half-melting the bonding metal layers bythermocompression, thereby electrically connection the conductors of thefilm carrier to the terminals of the electronic part.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a plan view and a sectional view of a film carrieraccording to one embodiment of the present invention;

FIGS. 2A and 2B are enlarged views showing two portions of the filmcarrier and a semiconductor element which undergo the bonding methodaccording to the present invention;

FIGS. 3A and 3B are a plan view and a sectional view of that portion ofthe film carrier which has been cut for bonding outer leads;

FIGS. 4A and 4B are a plan view and a sectional view of a printedcircuit board, with the portion of the film carrie bonded to the board;

FIGS. 5A to 5D are views of a film carrier according to one embodimentof the present invention; and

FIGS. 6A to 6D are plan views and sectional views of two type ofconductors of the film carrier according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

A film carrier according to the invention comprises a base film,conductors formed on the base film, and layers of a low-melting pointbonding metal, which are formed on the conductors. The film carrier ismounted on an electronic part, with the metal layers put in contact withthe terminals of the electronic part. The metal layers are heated to arelatively low temperature and thus half-melted, and then pressed ontothe terminals of the electronic part. Therefore, the numerous terminalscan be bonded at a time. Hence, only one alignment apparatus suffices tobond the terminals of the electronic part.

Preferably, the base film has a thickness of 50 to 150 μm, in particular100 to 130 μm in view of the required mechanical strength of the basefilm (i.e., the resistance to twisting and bending). On the base film,inner-lead sections and outer-lead sections are arranged atpredetermined intervals. These lead sections are formed in the followingway. Conductors, (usually copper foils) are bonded to the base film bymeans of an adhesive. Layers of a low-melting point bonding metal arebonded while being pressed onto the conductors. The metal layers areusually foils having a thickness of 50 to 150 μm. The metal layers areetched, and the conductors are then etched, thereby forming theinner-lead sections and the outer-lead sections. As a result, the filmcarrier is made.

The low-melting point metal can be any alloy, preferably low-meltingpoint adherent alloy, that can be bonded by thermocompression. Needlessto say, the lower the melting point, the better. The low-melting pointadherent alloy is a combination of a low-melting point solder alloy andan element which promotes the bonding at the interface between the alloylayers and the terminals of a semiconductor element. The solder alloy isa solder, such as Pb-Sn alloy or Sn-Zn alloy, made of two more metalsselected from the group consisting of Pb, Sn, Zn, Cd and Bi, andcontains In or another portion of Bi or Cd, in particular In, whichlowers the melting point of the solder. Sb can be used as the elementfor promoting the bonding at the interface between the alloy layers andthe terminals of the semiconductor element, or the terminals of aconductor. An element, such as Zn, Al, Ti, Si, Cr, Be or a rate earthelement, which has affinity with oxygen, can be added to Sb, therebyfurther promoting the bonding. The element which promotes the bondingcan contain other impurities as long as these impurities do not greatlyreduce the bonding ability of the alloy layers which characterize thepresent invention.

The low-melting point adherent alloy can be strongly bonded to glass,too, when an ultrasonic power is applied to it while it is in a moltencondition. Moreover, it can be firmly bonded by thermocompression in ahalf-molten state to glass, other oxides, and a metal such as Mo, Cr orTa which can hardly be wetted with a Pb-Sn solder. Therefore, it can beeasily bonded to metal oxides known as transparent electrode materials,such as SnO₂, In₂ O₃, and ITO (Indium Tin Oxide). The bonding mechanismof this alloy can be proved by the chemical bonding of(alloy)-(additive)-O-(oxide, etc.).

Generally, the terminals of semiconductor elements are made of aluminum.The surfaces of these aluminum terminals undergo natural oxidation, thusforming Al₂ O₃ layers, which are the prominent cause of an insufficientbonding between the terminals and the conductors. Nonetheless, the filmcarrier of this invention can bond the terminals to the conductors sincethe metal layers formed on the conductors has strong affinity withoxygen, even if the surface regions of the terminals are made of Al₂ O₃.Hence, when the film carrier is used, the aluminum terminals can bebonded to the conductor board, without being surface-treated (e.g.,deposition of a bonding metal). The film carrier of this invention can,therefore, facilitate the manufacture of apparatuses having integratedcircuits.

By using Pb, Sn, Zn, Sn and the like in an appropriate combination, thebonding metal can have a softening point of about 165° C. and a meltingpoint of 195° C. The present invention utilized the plastic deformationof the bonding alloy in a half-molten state to bond the terminals of asemiconductor element to the conductors of the film carrier. When thismetal is heated above its melting point, it will flow, possiblyshort-circuiting the adjacent terminals of the semiconductor element. Astrict temperature control must therefore be made in bonding theterminals to the conductors of the film carrier.

In the present invention, the bonding metal is in a half-moltencondition during the terminal-to-conductor bonding. It can beplastically deformed with a low pressure. Since no high pressure isrequired for the bonding, there is no possibility of damaging thesemiconductor element. In addition, since the bonding metal can behalf-melted at a relatively low temperature, there is little differencein temperature when cooled to room temperature, and the metal layershave no cracks after the terminal-to-conductor bonding. Therefore, thefilm carrier of this invention is suitable for use in bonding asemiconductor IC drive device to a liquid-crystal display having organiccolor filters. When this IC drive device is bonded to a transparentcircuit board, it must have transparent terminals made of SnO₂, In₂ O₃,or ITO.

Furthermore, since the bonding metal is in a half-molten condition whenthe conductors of the film carrier are pressed onto the terminals of thesemiconductor element, the terminals can be connected to the conductorseven if the terminals have different heights, and the gaps between theterminals, on the one hand, and the conductors, on the other hand, arenot equal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

FIG. 1A is a plan view of a film carrier according to the invention. Asis shown in this figure, the film carrier 2 comprises base film 21 madeof polyamide resin tape, and an inner-lead section 122 formed on basefilm 21, and an outer-lead section 123 also formed on base film 21.Sprocket holes 24 are cut in both longitudinal edges of base film 21.Inner-lead section 122 is to be connected to a semiconductor element.Outer-lead section 123 is to be connected to the external terminals ofthe electronic component. In FIG. 1A, square 131 represents the regionwhere the semiconductor element will be located, and square 132 showsthat portion of film carrier 2 which will be cut off in order to performouter-lead bonding. The polyamide resin can remain intact when heated upto about 160° C.

FIG. 1B is a sectional view, taken along line X--X in FIG. 1A. Squaresheet 125 of polyamide region, having a thickness of 35 μm, is adheredto base film 21, thus forming inner-lead section 122 projecting frombase film 21. Inner-lead section 122 can be formed by pressing anddeforming base film 21. Conductors 124 (i.e., copper strips), extendingparallel, are connected to sheet 125 by adhesive layer 26. Bonding metallayers 127 made of a low-melting point metal are bonded to conductors124 by thermocompression.

Conductors 124 and bonding metal layers 127 are formed by adhering afoil of the bonding metal to a foil of copper, and then photo-engravingthe bonding metal foil, thus forming strips, and finally photo-engravingthe upper foil, thereby forming conductors 124.

The bonding metal is a low-melting point adherent alloy consisting of20% by weight of Pb, 66% by weight of Sn, 10% by weight of In, 2% byweight of Sb, and 2% by weight of Zn. This alloy has a softening pointof 134° C. and a melting point of 160° C. Therefore, the conductors werebonded to the terminals of the semiconductor element at 150° C.

Adherent alloy layers 127 can be easily formed by etching the foil ofthe above-mentioned alloy, with hydrochloric acid or the like. In orderto protect copper conductors, a foil of an inert metal such as Cr, Mo orTi can be interposed between the copper foil and the adherent alloyfoil. Cr, Mo and Ti can strongly bond the copper foil and the bondingalloy foil.

In order to prevent copper conductors 124, made by etching selectedportions of the copper foil, from being oxidized, conductors 124 can beplated with Sn or Au layers having a thickness of 0.05 to 2 μm. Since Snor Au is thermally diffused into bonding metal layers 127 whenconductors 124 are pressed onto the terminals of a semiconductorelement, neither Sn nor Au layers adversely influences the electricalconnection of conductors 12 to the terminals of the semiconductorelement.

No metal layers for preventing the oxidation of bonding metal layers 127need be formed on layers 127 if the oxide layers are removed from layers127 immediately before copper conductors 124 are bonded to the terminalsof a semiconductor element. In order to remove the oxide layers frommetal layers 127, it suffices to etch layers 127 with dilutedhydrochloric acid, rinsing film carrier 2 with pure water, and dryingfilm carrier 2 by means of N₂ blowing.

It will now be explained how to bond inner-lead section 122 to theterminals of a semiconductor element. As is shown in FIG. 1B, aplurality of aluminum terminals 42 is formed on oxide layer 41 which inturn is formed on substrate 44 of semiconductor element 40. Functioncircuit 4 is formed in the surface of substrate 44 and covered withoxide layer 41. Aluminum terminals 42 are electrically connected to thisfunction circuit 43. To bond bonding metal layers 127 of film carrier 2to terminals 42 of semiconductor element 40, layers 127 are aligned withterminals 42, respectively. Then, tool 32 is moved down in the directionof arrow 4, thereby pressing film carrier 2 onto semiconductor element40 with a pressure of 1 to 10 g for each 100 μm×100 μm terminal 42,while bonding metal layers 172 are being heated to 150° C., as isillustrated in FIG. 2A.

As is shown in FIG. 2A, semiconductor element 40 is held on jig 130.This jig 130 is heated to 100° to 120° C. The heat is transmitted fromjig 130 to bonding metal layers 127 through substrate 44 and terminals42, whereby bonding metal layers 127 are heated. Therefore, thetemperature of layers 127 rises to 145° to 155° C. within a few secondswhen the heater (not shown) provided within tool 32 is turned on. Whenbonding metal layers 127 are heated to 145° to 155° C., the heater isturned off. Then, layers 127 are cooled to a temperature below thesoftening point (134° C.) of the bonding alloy, while maintaining layers127 pressed onto terminals 42 with a pressure of 10 to 100 kg/cm²Thereafter, tool 32 is lifted from film carrier 2. In the case where Auor Sn layers are plated on bonding metal layers 127, these plated layersare diffused into layers 127 while layers 127 are being bonded toterminals 42 of the semiconductor element.

As is illustrated in FIG. 2B, that portion of each bonding metal layer127, which is interposed between terminal 42 and that portion of copperconductor 124 which is located below sheet 125, undergoes elasticdeformation. Conductors 124 can thus be bonded to terminals 42 at arelatively low temperature under a relatively low pressure.

In order to use the film carrier of this invention in outer bonding,each portion of the film carrier, to which a semiconductor element hasbeen inner-bonded, is cut out. Then, if necessary, the outer-leadsection of this carrier portion is trimmed, and the terminals of thesemiconductor element are also trimmed; that is, so-called "leadforming" is carried out.

FIG. 3A is a plan view of one portion of the film carrier, with onesemiconductor element bonded to it. FIG. 3B is a sectional view takenalong line Y--Y in FIG. 3A.

The outer-lead bonding is performed using the same steps as in theinner-lead bonding. As is shown in FIG. 4A, conductors 31 are formed onprinted circuit board 3. Conductors 124 formed on outer-lead section 123of film carrier 2 are aligned with these conductors 31, and then arebonded to conductors 31 by thermocompression. FIG. 4B is a sectionalview taken along line Z--Z in FIG. 4A. As this figure clearly shows,semiconductor element 40 is provided in recess 32 cut in printed circuitboard 3.

In order to protect the connection between the terminals ofsemiconductor element 40 and the conductors 124 of film carrier 2, andalso to enhance the durability of the film carrier, film carrier 2 canbe covered with a coating or a film of, for example, polyester, exceptfor inner-lead section 122 and outerlead section 123. It suffices ifthis coating or film has a thickness of 20 to 50 μm.

The conductors formed on base film 21 can be made of the low-meltingpoint bonding metal. If this is the case, the bonding metal layers areformed directly on base film 21.

EXAMPLE 2

FIG. 5A is a plan view of a film carrier according to the invention. Asis shown in this figure, the film carrier 302 comprises base film 321made of polyimide resin tape, and an inner-lead section 322 cut insidebase film 321, and an outer-lead section 323 also cut inside base film321. Sprocket holes 320 are cut in both longitudinal edges of base film321. Inner-lead section 322 is to be connected to a semiconductorelement. Outer-lead section 323 is to be connected to the externalterminals of the driving circuit in the liquid crystal display device.In FIG. 5A, square 331 represents the region where the semiconductorelement will be located, and hexagon 332 shows that portion of filmcarrier 320 which will be cut off in order to perform outer-leadbonding. The polyamide resin can remain intact when heated up to about160° C.

FIG. 5B is a sectional view, taken along line W--W in FIG. 5A.Conductors 324 (i.e., copper strips) are connected to base film 321 byadhesive layer 326. Bonding metal layers 327 made of a low-melting pointmetal are bonded to conductors 324 by thermocompression.

Conductors 324 and bonding metal layers 327 are formed by adhering afoil of the bonding metal to a foil of copper, and then photo-engravingthe bonding metal foil, thus forming strips, and finally photo-engravingthe upper foil, thereby forming conductors 324.

The bonding metal of this Example was the same as that of Example 1.

It will no be explained how to bond inner-lead section 322 to theterminals of a semiconductor element. As is shown in FIG. 5B, aplurality of aluminum terminals 342 is formed on oxide layer 341 whichin turn is formed on substrate 344 of semiconductor element 340.Function circuit 343 is formed in the surface of substrate 344 andcovered with oxide layer 341. Aluminum terminals 342 are electricallyconnected to this function circuit 343. To bond bonding metal layers 327of film carrier 302 to terminals 342 of semiconductor element 340,layers 327 are aligned with terminals 342, respectively. Then, tool 332is moved down in the direction of arrow 304, thereby pressing filmcarrier 302 onto semiconductor element 340 with a pressure of 1 to 10 gfor each 100 μm×100 μm terminal 342, while bonding metal layers 372 arebeing heated to 150° C.

In order to use the film carrier of this invention in outer bonding,each portion of the film carrier, to which a semiconductor element hasbeen inner-bonded, is cut out. Then, if necessary, the outer-leadsection of this carrier portion is trimmed, and the terminals of thesemiconductor element are also trimmed; that is, so-called "leadforming" is carried out.

FIG. 5C is a plan view of one portion of the film carrier, with onesemiconductor element bonded to it.

The outer-lead bonding is performed in the same steps as in theinner-lead bonding. As is shown in FIG. 5D, conductors 324 are connectedto the conductors 352 arrayed on liquid crystal display device 353having display portion 355. Conductors 324 formed on outer-lead section323 of film carrier 302 are aligned with these conductor 352, and thenare bonded to conductors 352 by thermocompression.

Input terminal of semiconductor device 329 is bonded to wiring 356 onglass-epoxy board 354.

EXAMPLE 3

The shape of the conductors of the film carrier according to the presentinvention will be described. FIG. 6A is a plan view of one of conductors200 made of copper foils. As is shown in this figure, conductor 200consists of portions A, B, and C. Portion A will be bonded to, forexample, an IC chip. Portion B connects portion A to portion C. FIG. 6Bis a sectional view of conductor 200. As can be clearly shown in FIG.6B, h_(B) <h_(C) ≦h_(A), where h_(A), h_(B), and h_(C) are the heightsof portions A, B, and C, respectively. Since portion B is shorter thanportions A and C, portion B does not adversely influence the bondingbetween portion A and the IC chip. If conductor 200 is made from a striphaving a uniform thickness, and portion B has width W_(B) less thanwidth W_(A) of portion A, h_(B) will automatically become less thanh_(A) due to the undercutting when said strip is etched.

In order to make height h_(B) of portion B less than h_(A) and h_(C) asis shown in FIG. 6D, portion B of a strip having a uniform width (W_(A)=W_(B) =W_(C)) can be pressed as is shown in FIG. 6C. In this case,portion B will become broader than portions A and C after it has beenpressed.

In the embodiments described above, the conductors of the film carrierare bonded to the terminals of the semiconductor element in atmosphere.Instead, the bonding can be carried out in an inert-gas atmosphere. Ifthe bonding is performed in an inert-gas atmosphere, the fine andslender conductors of the film carrier and the fine and thin terminalsof the semiconductor element will be protected against oxidation.

What is claimed is:
 1. A film carrier comprising:a base film; aplurality of conductors formed on base film, and a plurality of layersformed on the conductors, each being made of a bonding metal having amelting point of approximately 195° C. and a softening point ofapproximately 165° C.
 2. The film carrier according to claim 1, whereineach of said layers of the bonding metal is formed on at least one endportion of the corresponding conductor.
 3. The film carrier according toclaim 2, wherein said end portion of the conductor is a bonding section.4. The film carrier according to claim 3, wherein the bonding section ofsaid conductor is an inner-lead section or an outer-lead section.
 5. Thefilm carrier according to claim 4, wherein the inner-lead section ofsaid conductor projects from said base film.
 6. The film carrieraccording to claim 4, wherein the following relation is established:

    h.sub.B <h.sub.C ≦h.sub.A

where h_(A) is the height of said one end portion of said conductor,h_(B) is the height of the non-bonding section of said conductor andh_(C) is the height of the portion of said conductor which does notcorrespond to the inner-lead section or outer-lead section.
 7. The filmcarrier according to claim 1, wherein said bonding metal is alow-melting point adherent alloy.
 8. The film carrier according to claim7, wherein said adherent alloy comprises a low-melting point solderalloy, and an element which promotes the bonding at the interfacebetween the alloy layers and the terminals of an electronic part.
 9. Thefilm carrier according to claim 8, wherein said solder alloy comprisesIn and two elements selected from the group consisting of Pb, Sn, Zn,Cd, and Bi.
 10. The film carrier according to claim 8, wherein saidelement which promotes the bonding at the interface is Sb.
 11. The filmcarrier according to claim 7, wherein said adherent alloy comprises alow-melting point solder alloy, an element which promotes the bonding atthe interface between the alloy layers and the terminals of anelectronic part, and an element which has a prominent affinity withoxygen.
 12. The film carrier according to claim 11, wherein said solderalloy comprises In and two elements selected from the group consistingof Pb, Sn, Zn, Cd, and Bi.
 13. The film carrier according to claim 11,wherein said element which promotes the bonding at the interface is Sb.14. The film carrier according to claim 11, wherein said element whichhas a prominent affinity with oxygen is one or two elements selectedfrom the group consisting of Zn, Al, Ti, Si, Cr, Be, and a rare earthmetal.
 15. The film carrier according to claim 1, wherein said bondingmetal layers are used as said conductors.
 16. A film carrier accordingto claim 11 wherein the material which forms the terminal of theelectronic part is selected from the group consisting of Al and ITO(Indium Tin Oxide).
 17. A bonding method comprising the stepsof:positioning a film carrier having a base film, a plurality ofconductors formed on the base film, and a plurality of bonding metallayers formed on the conductors, each said bonding metal layer beingmade of a bonding metal having a melting point of approximately 195° C.and a softening point of approximately 165° C., so that said bondingmetal layers align with the terminals of an electronic part; halfmelting said bonding metal layers at a temperature so that said bondingmetal layers are neither liquid nor solid compression bonding saidheated bonding metal layers to said terminal of an electronic part, saidelectronic part being in solid form, thereby electrically connecting theconductors of said film carrier to the terminals of the electronic part.18. The bonding method according to claim 17, wherein those potions ofsaid conductors on which said bonding metal layers are formed constitutean inner-lead section which projects from said base film.